A 39-year-old man with a diffuse axonal injury self-presented to the cognitive function clinic of the Ashikaga Red Cross Hospital complaining of behavioral errors in his daily life, such as scooping hot rice into a glass instead of a bowl or forgetting to turn off the gas stove after cooking. This type of error has been referred to as an action slip-a form of unintentional behavioral error that occurs when an individual attempts to perform an automatic and/or familiar task. In this case, action slips occurred nine times a day on average and had a serious and long-term impact on the man's daily quality of life. To reduce the impact of action slips, we created a one-on-one cognitive intervention that used a combination of mnemonic strategies, such as verbalizing his actions as he carried them out, and external aids, including a waist pouch, a voice-controlled artificial intelligence (AI) speaker, and an AI key finder. After 3 years of intervention, the man reported some improvement in his daily activities and a reduction in the number of action slips. Thus, intervention strategies for individuals with a diffuse axonal injury may benefit from targeting action slips. To our knowledge, this is the first detailed description of action slips in an individual with a diffuse axonal injury.